Just like we prepare for winter, trees make their own preparations. Rather than layering up with dozens of fabrics they follow the same principle of hibernation a bear follows, that is the conservation of his energy.
During the winter season in a particular hemisphere, the earth receives less sunlight than usual, sunlight which is paramount for leaf growth and sustenance for the trees is only available in a scarce amount. The shedding of leaves is a coping mechanism adapted by these deciduous trees. Generally, leaves spread outward towards where they can collect the maximum amount of sunlight, which when absorbed can run the age old process of photosynthesis. This is the conversion of sunlight and moisture to useful energy.
But there is little sunlight to go around in the winter and trees can sense this beforehand and start the shedding process. The process goes through two seasons, which is why before turning bare trees give us the spectacular color shows of the year, turning from green to yellow to orange and finally completely falling off. The colors of the leaves are a choice made by the trees to safeguard itself from insect attack.
Trees like all living organisms make decisions for their survival taking in the pros and cons at every step of the way. The idea behind shedding leaves is no different as it is simply more cost effective and energy efficient to shed them, rather than to let them starve for what little sunlight they may receive. Trees have to conserve energy nutrients and moisture thorough out the winter and leaves unfortunately make use of a lot of nutrients and lose quite a lot of water through their pores, therefore, they are sacrificed for the greater benefit. It really is all about survival out there.